1. Technical Field
The present invention relates to a magnetic shielding apparatus and a magnetic shielding method.
2. Related Art
Diagnostic imaging is widely used in medical practice. The diagnostic imaging, which is non-invasive and therefore puts less burden on the human body, is preferably used especially in examining organs important for the vital activities, such as the heart or brain.
The organs of a living body always generate weak currents due to the activities of neurons. By measuring magnetic fields caused by the currents, the condition of the organ can be known.
Specifically, the magnetic field arising from the living body is extremely weak such as from about 0.1 picotesla (1×10−13 T) to about 10 picotesla (1×10−11 T).
One of methods to detect and image such a magnetic field is magnetic source imaging. For example, a method of recording magnetic fields formed by the activities of the heart is known as magnetocardiography (MCG). Moreover, a method of measuring and recording magnetic fields formed by the activities of the brain is known as magnetoencephalography (MEG).
On the other hand, an external magnetic field such as terrestrial magnetism is, for example, from about 10 microtesla (1×10−5 T) to about 100 microtesla (1×10−4 T), and has a size of 100,000 times or greater compared to the magnetic field generated from the brain or heart. Therefore, such an external magnetic field becomes noise in highly sensitive magnetic field measurement.
That is, for detecting a weak magnetic field from a living body, an external magnetic field serving as noise needs to be blocked in addition to the use of a highly sensitive magnetic field measuring device. For blocking the external magnetic field, a magnetic shielding apparatus needs to be used. Then, as the magnetic shielding apparatus to block the external magnetic field, techniques have been known as disclosed in, for example, JP-A-6-167583, JP-A-2002-94280, JP-A-2002-257914, JP-A-2005-294537, JP-A-2007-129049, JP-A-2008-282983, JP-A-2009-175067, and Japanese Patent No. 4377566 (Patent Document 1 to Patent Document 8).
According to the techniques disclosed in Patent Document 1 to Patent Document 8, a canceling magnetic field is generated so as to minimize the magnetic field of a space (space where a magnetic field measuring device is arranged) as a control object in the interior of the magnetic shielding apparatus, or a canceling magnetic field is generated so as to minimize an inflowing magnetic field into an opening or the like of the magnetic shielding apparatus.
However, in the techniques disclosed in Patent Document 1 to Patent Document 8, there is a problem that it is difficult to create an environment where a magnetic field gradient is extremely gentle in the interior of a magnetic shield. Specifically, a magnetic field from the outside can be reduced in the related art; however, there is a problem that a gradient is generated in the magnetic field state of the interior of the magnetic shield because a magnetization component or the like of the magnetic shield is not considered.